Highly controlled physicochemical surface properties are key factors in the development and optimization of devices for different fields, ranging from bio-materials to packaging to automotive applications. A possible way of tailoring the surface chemistry without influencing other surface parameters, such as roughness and optical properties, is to use very thin films, in particular monomolecular adlayers. The general functionality of these molecular systems is three-fold: a binding sequence for the immobilization on the surface, a spacer that often contributes to the functional properties of the surface (stability, diffusion barrier, protein resistance) and a chemical group that presents a particular (bio-)functional moiety. Different combinations of amphiphiles/substrates have been developed and successfully applied in the past years, e.g. alkanethiols on gold or silver,1, 2 silanes on a variety of substrates,3-8 alkane phosph(on)ates on metal oxides9-13 or Pluronics on hydrophobic substrates.14 
In terms of surface functionalization for biomedical applications, poly(ethylene glycol) has been widely investigated thanks to its non-fouling, protein-resistant properties.15 Graft co-polymers have been extensively investigated in recent years at the Laboratory for Surface Science and Technology (LSST) of the Swiss Federal Institute of Technology Zurich and today, PEG-graft polyelectrolytes such as poly(L-Lysine)-grafted-poly(ethylene glycol) (PLL-g-PEG) adlayers have become a standard tool for surface modifications of charged substrates. The design of this system is based on a positively charged backbone (PLL, pKa=10.5) and a brush of PEG side chains. Once adsorbed, the backbone forms a stable electrostatic bond to the negatively charged metal-oxide substrate, while the side chains form a dense, closely packed, protein-resistant adlayer towards the biological environment. The possibility of varying the PEG density by changing the ratio between PEG side chains and Lys units,15 as well as the possibility of adding bio-functionally active moieties such as small proteins16 or short peptide sequences at the PEG-omega position,17 makes this systems highly versatile for the development of novel biosensors and biomaterial surfaces.
However, this and the other assembly systems, have limitations in terms of substrate choice, as well as binding strength. Hitherto, only one system has been proven to be able to adsorb and form a stable, nearly covalent bond to almost all types of surfaces (metals,18, 19 stainless steel,20 metal oxide or ceramics21, 22 and polymers23-28). This system is based on a non-proteinogenic amino acid: 3,4-dihydroxyphenylalanine (DOPA). Indeed, marine mussels, for example, are known to secrete a number of unique proteins that mediate adhesion to the variety of underwater substrates on which they reside.29 These so-called mussel-adhesive proteins contain an unusually high concentration of 3,4-dihydroxyphenylalanine (DOPA), which is believed to impart the strong adhesion characteristic of these proteins.19, 29, 30 Furthermore, Messersmith and coworkers have exploited these adhesive characteristics by coupling DOPA-containing peptides to PEG, and have recently demonstrated that these molecules are capable of modifying metals, metal oxides, semiconductor and polymer surfaces, rendering them resistant to cell adhesion and spreading.19, 31, 32 The DOPA adhesion mechanism is proposed to function either through chelation of surface bound metal centers or via oxidative crosslinking of DOPA.20, 33, 34 
In WO 03/008376 it has already been proposed to either couple DOPA to a polymer via an urethane bond or via an ester or an amide bond. In case of attachment via a urethane bond, the carboxylic group can be trans-formed into an ester group.
DOPA analogs are also found in other organisms, such as cyanobacteria. These organisms have faced the challenge of iron acquisition for over 2.5 billion years, and have met this task by developing sophisticated strategies. A key substance is the growth factor anachelin35, 36 which was evolutionary optimized for binding to Fe(III). This molecule contains a chromophore that is biosynthetically derived from DOPA and contains a positive charge.
Although DOPA including polymers have been found to result in adlayers sufficiently stable for several applications, there is still a need for molecules that are more generally applicable and/or more stable in solution and/or form more stable adlayers.